Re: [PATCH v3 0/6] implement KASLR for powerpc/fsl_booke/64

[Daniel Axtens]

Jason Yan <yanaijie@xxxxxxxxxx> writes:

> Hi Daniel,
>
> å 2020/2/26 15:16, Daniel Axtens åé:
>> Hi Jason,
>> 
>>> This is a try to implement KASLR for Freescale BookE64 which is based on
>>> my earlier implementation for Freescale BookE32:
>>> https://patchwork.ozlabs.org/project/linuxppc-dev/list/?series=131718
>>>
>>> The implementation for Freescale BookE64 is similar as BookE32. One
>>> difference is that Freescale BookE64 set up a TLB mapping of 1G during
>>> booting. Another difference is that ppc64 needs the kernel to be
>>> 64K-aligned. So we can randomize the kernel in this 1G mapping and make
>>> it 64K-aligned. This can save some code to creat another TLB map at
>>> early boot. The disadvantage is that we only have about 1G/64K = 16384
>>> slots to put the kernel in.
>>>
>>>      KERNELBASE
>>>
>>>            64K                     |--> kernel <--|
>>>             |                      |              |
>>>          +--+--+--+    +--+--+--+--+--+--+--+--+--+    +--+--+
>>>          |  |  |  |....|  |  |  |  |  |  |  |  |  |....|  |  |
>>>          +--+--+--+    +--+--+--+--+--+--+--+--+--+    +--+--+
>>>          |                         |                        1G
>>>          |----->   offset    <-----|
>>>
>>>                                kernstart_virt_addr
>>>
>>> I'm not sure if the slot numbers is enough or the design has any
>>> defects. If you have some better ideas, I would be happy to hear that.
>>>
>>> Thank you all.
>>>
>> 
>> Are you making any attempt to hide kernel address leaks in this series?
>
> Yes.
>
>> I've just been looking at the stackdump code just now, and it directly
>> prints link registers and stack pointers, which is probably enough to
>> determine the kernel base address:
>> 
>>                    SPs:               LRs:             %pS pointer
>> [    0.424506] [c0000000de403970] [c000000001fc0458] dump_stack+0xfc/0x154 (unreliable)
>> [    0.424593] [c0000000de4039c0] [c000000000267eec] panic+0x258/0x5ac
>> [    0.424659] [c0000000de403a60] [c0000000024d7a00] mount_block_root+0x634/0x7c0
>> [    0.424734] [c0000000de403be0] [c0000000024d8100] prepare_namespace+0x1ec/0x23c
>> [    0.424811] [c0000000de403c60] [c0000000024d7010] kernel_init_freeable+0x804/0x880
>> 
>> git grep \\\"REG\\\" arch/powerpc shows a few other uses like this, all
>> in process.c or in xmon.
>> 
>
> Thanks for reminding this.
>
>> Maybe replacing the REG format string in KASLR mode would be sufficient?
>> 
>
> Most archs have removed the address printing when dumping stack. Do we 
> really have to print this?
>
> If we have to do this, maybe we can use "%pK" so that they will be 
> hidden from unprivileged users.

I suspect that you will find it easier to convince people to accept a
change to %pK than removal :)

BTW, I have a T4240RDB so I might be able to test this series at some
point - do I need an updated bootloader to pass in a random seed, or is
the kernel able to get enough randomness by itself? (Sorry if this is
explained elsewhere in the series, I have only skimmed it lightly!)

Regards,
Daniel
>
> Thanks,
> Jason
>
>> Regards,
>> Daniel
>> 
>> 
>>> v2->v3:
>>>    Fix build error when KASLR is disabled.
>>> v1->v2:
>>>    Add __kaslr_offset for the secondary cpu boot up.
>>>
>>> Jason Yan (6):
>>>    powerpc/fsl_booke/kaslr: refactor kaslr_legal_offset() and
>>>      kaslr_early_init()
>>>    powerpc/fsl_booke/64: introduce reloc_kernel_entry() helper
>>>    powerpc/fsl_booke/64: implement KASLR for fsl_booke64
>>>    powerpc/fsl_booke/64: do not clear the BSS for the second pass
>>>    powerpc/fsl_booke/64: clear the original kernel if randomized
>>>    powerpc/fsl_booke/kaslr: rename kaslr-booke32.rst to kaslr-booke.rst
>>>      and add 64bit part
>>>
>>>   .../{kaslr-booke32.rst => kaslr-booke.rst}    | 35 +++++++--
>>>   arch/powerpc/Kconfig                          |  2 +-
>>>   arch/powerpc/kernel/exceptions-64e.S          | 23 ++++++
>>>   arch/powerpc/kernel/head_64.S                 | 14 ++++
>>>   arch/powerpc/kernel/setup_64.c                |  4 +-
>>>   arch/powerpc/mm/mmu_decl.h                    | 19 ++---
>>>   arch/powerpc/mm/nohash/kaslr_booke.c          | 71 +++++++++++++------
>>>   7 files changed, 132 insertions(+), 36 deletions(-)
>>>   rename Documentation/powerpc/{kaslr-booke32.rst => kaslr-booke.rst} (59%)
>>>
>>> -- 
>>> 2.17.2
>> 
>> .
>>